Understanding the impact of modiolus porosity on stimulation of spiral ganglion neurons by cochlear implants

Moderate-to-profound sensorineural hearing loss in humans is treatable by electrically stimulating the auditory nerve (AN) with a cochlear implant (CI). In the cochlea, the modiolus presents a porous bony interface between the CI electrode and the AN. New bone growth caused by the presence of the CI electrode or neural degeneration inflicted by ageing or otological diseases might change the effective porosity of the modiolus and, thereby, alter its electrical material properties. Using a volume conductor description of the cochlea, with the aid of a 'mapped conductivity' method and an ad-hoc 'regionally kinetic' equation system, we show that even a slight variation in modiolus porosity or pore distribution can disproportionately affect AN stimulation. Hence, because of porosity changes, an inconsistent CI performance might occur if neural degeneration or new bone growth progress after implantation. Appropriate electrical material properties in accordance with modiolar morphology and pathology should be considered in patient-specific studies. The present first-of-its-kind in-silico study advocates for contextual experimental studies to further explore the utility of modiolus porous morphology in optimising the CI outcome.

Auditory and speech performance after unilateral cochlear implantation for cochlear nerve canal stenosis

To explore the correlation between the width of the bony cochlear nerve canal (CNC) and long-term auditory rehabilitation after unilateral cochlear implantation (CI) in pediatric patients with congenital deafness and bilateral cochlear nerve canal stenosis (CNCS). A retrospective review was performed on 10 patients with bilateral CNCS and bilateral congenital profound hearing loss who each underwent unilateral cochlear implantation. The width of the CNC was determined on computed tomography (CT) imaging and following CI, auditory and speech performance following CI were graded using categories of auditory performance (CAP), speech intelligibility rating (SIR), and the meaningful auditory integration scale (MAIS) at 24 months following implantation. No correlation was noted between CAP score and CNCS at 24 months post CI (P > .05). A positive correlation was noted between SIR score and CNC width (ρ = .81, P < .05). Similarly, a positive correlation was noted between MAIS and CNC width (ρ = .71, P < .05). The width of the CNC in patients with CNCS is positively correlated with some long-term auditory and speech outcomes after CI.

Predicting early auditory evoked potentials using a computational model of auditory-nerve processing

Non-invasive electrophysiological measures, such as auditory evoked potentials (AEPs), play a crucial role in diagnosing auditory pathology. However, the relationship between AEP morphology and cochlear degeneration remains complex and not well understood. Dau [J. Acoust. Soc. Am. 113, 936-950 (2003)] proposed a computational framework for modeling AEPs that utilized a nonlinear auditory-nerve (AN) model followed by a linear unitary response function. While the model captured some important features of the measured AEPs, it also exhibited several discrepancies in response patterns compared to the actual measurements. In this study, an enhanced AEP modeling framework is presented, incorporating an improved AN model, and the conclusions from the original study were reevaluated. Simulation results with transient and sustained stimuli demonstrated accurate auditory brainstem responses (ABRs) and frequency-following responses (FFRs) as a function of stimulation level, although wave-V latencies remained too short, similar to the original study. When compared to physiological responses in animals, the revised model framework showed a more accurate balance between the contributions of auditory-nerve fibers (ANFs) at on- and off-frequency regions to the predicted FFRs. These findings emphasize the importance of cochlear processing in brainstem potentials. This framework may provide a valuable tool for assessing human AN models and simulating AEPs for various subtypes of peripheral pathologies, offering opportunities for research and clinical applications.

Evolution of air-borne vocalization: Insights from neural studies in the archeobatrachian species Bombina orientalis

Vocalization of tetrapods evolved as an air-driven mechanism. Thus, it is conceivable that the underlaying neural network might have evolved from more ancient respiratory circuits and be made up of homologous components that generate breathing rhythms across vertebrates. In this context, the extant species of stem anurans provide an opportunity to analyze the connection of the neural circuits of lung ventilation and vocalization. Here, we analyzed the fictive lung ventilation and vocalization behavior of isolated brains of the Chinese fire-bellied toad Bombina orientalis during their mating season by nerve root recordings. We discovered significant differences in durations of activation of male brains after stimulation of the statoacoustic nerve or vocalization-relevant forebrain structures in comparison to female brains. The increased durations of motor nerve activities in male brains can be interpreted as fictive calling, as male's advertisement calls in vivo had the same general pattern compared to lung ventilation, but longer duration periods. Female brains react to the corresponding stimulations with the same shorter activity pattern that occurred spontaneously in both female and male brains and thus can be interpreted as fictive lung ventilations. These results support the hypothesis that vocal circuits evolved from ancient respiration networks in the anuran caudal hindbrain. Moreover, we could show that the terrestrial stem archeobatrachian Bombina spec. is an appropriate model to study the function and evolution of the shared network of lung ventilation and vocal generation.

Drop Attack Treated by Microvascular Decompression of the Superior Vestibular Nerve

We report a case of cochleovestibular neurovascular compressive syndrome (CVCS)-induced drop attack treated with microvascular decompression (MVD) of the superior vestibular nerve. This report discusses the merits of surgical intervention through a review of related literature. A 58-year-old woman was referred to our clinic with a chief complaint of intermittent, strong, right-sided tinnitus lasting for a few seconds immediately prior to drop attack. Magnetic resonance imaging (MRI) showed bilateral neurovascular contact between the anterior inferior cerebellar artery (AICA) and the vestibulocochlear nerve. Based on MRI findings, history of present illness, and response to anticonvulsants, CVCS was suspected, and surgical decompression on the right side was subsequently performed. The patient became asymptomatic immediately after the surgery, and the vestibular-evoked myogenic potentials were normalized. No recurrence was reported during a 1-year follow-up period.

Neural Fluctuation Contrast as a Code for Complex Sounds: The Role and Control of Peripheral Nonlinearities

The nonlinearities of the inner ear are often considered to be obstacles that the central nervous system has to overcome to decode neural responses to sounds. This review describes how peripheral nonlinearities, such as saturation of the inner-hair-cell response and of the IHC-auditory-nerve synapse, are instead beneficial to the neural encoding of complex sounds such as speech. These nonlinearities set up contrast in the depth of neural-fluctuations in auditory-nerve responses along the tonotopic axis, referred to here as neural fluctuation contrast (NFC). Physiological support for the NFC coding hypothesis is reviewed, and predictions of several psychophysical phenomena, including masked detection and speech intelligibility, are presented. Lastly, a framework based on the NFC code for understanding how the medial olivocochlear (MOC) efferent system contributes to the coding of complex sounds is presented. By modulating cochlear gain control in response to both sound energy and fluctuations in neural responses, the MOC system is hypothesized to function not as a simple feedback gain-control device, but rather as a mechanism for enhancing NFC along the tonotopic axis, enabling robust encoding of complex sounds across a wide range of sound levels and in the presence of background noise. Effects of sensorineural hearing loss on the NFC code and on the MOC feedback system are presented and discussed.

Comparing pronunciation challenges in South Korean preschoolers with unilateral single-sided deafness due to cochlear nerve deficiency to a norm-referenced standard

This study aimed to compare the development of pronunciation in South Korean preschoolers with unilateral cochlear nerve deficiency (CND) to that of age-matched preschoolers with normal hearing, a topic that has not been explored previously. In a retrospective analysis, 25 preschoolers with unilateral CND who had undergone a speech evaluation battery, including a pronunciation and vocabulary test, were enrolled. Utilizing the Urimal Test of Articulation and Phonation and customized language ability tests, pronunciation and vocabulary were assessed. The subjects' speech evaluation scores were converted into age-adjusted z-scores using normal controls' data. While vocabulary performance was within normal limits, their average pronunciation z-score was -2.90, significantly lower than both the zero reference point and their vocabulary z-scores. None of the subjects scored above average in pronunciation. Thirteen patients were recommended for articulation therapy, seven were considered as potential candidates for this therapy, and the remaining five were within normal limits. There was no observed correlation between the development of pronunciation and vocabulary. Notably, some subjects' pronunciation scores did not improve, even after serial follow-up during their preschool years. Despite typical vocabulary development, preschoolers with unilateral CND exhibit significant delays in pronunciation. These findings emphasize the necessity for vigilant monitoring of their language development.

Estimation of Cochlear Frequency Selectivity Using a Convolution Model of Forward-Masked Compound Action Potentials

PURPOSE

Frequency selectivity is a fundamental property of the peripheral auditory system; however, the invasiveness of auditory nerve (AN) experiments limits its study in the human ear. Compound action potentials (CAPs) associated with forward masking have been suggested as an alternative to assess cochlear frequency selectivity. Previous methods relied on an empirical comparison of AN and CAP tuning curves in animal models, arguably not taking full advantage of the information contained in forward-masked CAP waveforms.

METHODS

To improve the estimation of cochlear frequency selectivity based on the CAP, we introduce a convolution model to fit forward-masked CAP waveforms. The model generates masking patterns that, when convolved with a unitary response, can predict the masking of the CAP waveform induced by Gaussian noise maskers. Model parameters, including those characterizing frequency selectivity, are fine-tuned by minimizing waveform prediction errors across numerous masking conditions, yielding robust estimates.

RESULTS

The method was applied to click-evoked CAPs at the round window of anesthetized chinchillas using notched-noise maskers with various notch widths and attenuations. The estimated quality factor Q10 as a function of center frequency is shown to closely match the average quality factor obtained from AN fiber tuning curves, without the need for an empirical correction factor.

CONCLUSION

This study establishes a moderately invasive method for estimating cochlear frequency selectivity with potential applicability to other animal species or humans. Beyond the estimation of frequency selectivity, the proposed model proved to be remarkably accurate in fitting forward-masked CAP responses and could be extended to study more complex aspects of cochlear signal processing (e.g., compressive nonlinearities).

Mechanisms of age-related hearing loss at the auditory nerve central synapses and postsynaptic neurons in the cochlear nucleus

Sound information is transduced from mechanical vibration to electrical signals in the cochlea, conveyed to and further processed in the brain to form auditory perception. During the process, spiral ganglion neurons (SGNs) are the key cells that connect the peripheral and central auditory systems by receiving information from hair cells in the cochlea and transmitting it to neurons of the cochlear nucleus (CN). Decades of research in the cochlea greatly improved our understanding of SGN function under normal and pathological conditions, especially about the roles of different subtypes of SGNs and their peripheral synapses. However, it remains less clear how SGN central terminals or auditory nerve (AN) synapses connect to CN neurons, and ultimately how peripheral pathology links to structural alterations and functional deficits in the central auditory nervous system. This review discusses recent progress about the morphological and physiological properties of different subtypes of AN synapses and associated postsynaptic CN neurons, their changes during aging, and the potential mechanisms underlying age-related hearing loss.

Insights Into Electrophysiological Metrics of Cochlear Health in Cochlear Implant Users Using a Computational Model

PURPOSE

The hearing outcomes of cochlear implant users depend on the functional status of the electrode-neuron interface inside the cochlea. This can be assessed by measuring electrically evoked compound action potentials (eCAPs). Variations in cochlear neural health and survival are reflected in eCAP-based metrics. The difficulty in translating promising results from animal studies into clinical use has raised questions about to what degree eCAP-based metrics are influenced by non-neural factors. Here, we addressed these questions using a computational model.

METHODS

A 2-D computational model was designed to simulate how electrical signals from the stimulating electrode reach the auditory nerve fibers distributed along the cochlea, evoking action potentials that can be recorded as compound responses at the recording electrodes. Effects of physiologically relevant variations in neural survival and in electrode-neuron and stimulating-recording electrode distances on eCAP amplitude growth functions (AGFs) were investigated.

RESULTS

In line with existing literature, the predicted eCAP AGF slopes and the inter-phase gap (IPG) effects depended on the neural survival, but only when the IPG effect was calculated as the difference between the slopes of the two AGFs expressed in linear input-output scale. As expected, shallower eCAP AGF slopes were obtained for increased stimulating-recording electrode distance and larger eCAP thresholds for greater electrode-neuron distance. These non-neural factors had also minor interference on the predicted IPG effect.

CONCLUSIONS

The model predictions demonstrate previously found dependencies of eCAP metrics on neural survival and non-neural aspects. The present findings confirm data from animal studies and provide insights into applying described metrics in clinical practice.

Auditory robustness and resilience in the aging auditory system of the desert locust

After overexposure to loud music, we experience a decrease in our ability to hear (robustness), which usually recovers (resilience). Here, we exploited the amenable auditory system of the desert locust, Schistocerca gregaria, to measure how robustness and resilience depend on age. We found that gene expression changes are dominated by age as opposed to noise exposure. We measured sound-evoked nerve activity for young and aged locusts directly, after 24 hours and 48 hours after noise exposure. We found that both young and aged locusts recovered their auditory nerve function over 48 hours. We also measured the sound-evoked transduction current in individual auditory neurons, and although the transduction current magnitude recovered in the young locusts after noise exposure, it failed to recover in the aged locusts. A plastic mechanism compensates for the decreased transduction current in aged locusts. We suggest key genes upregulated in young noise-exposed locusts that mediate robustness to noise exposure and find potential candidates responsible for compensatory mechanisms in the auditory neurons of aged noise-exposed locusts.

[Intraoperative cochlear nerve monitoring in microvascular decompression of hemifacial spasm and resection of vestibular schwannoma]

Objective: To analyze the application of direct cochlear nerve monitoring technology-cochlear nerve action potential (CNAP) monitoring in resection of vestibular schwannoma (VS) and to compare with which in microvascular decompression (MVD) of hemifacial spasm (HFS), in order to provide reference for identification of the cochlear nerve during VS resection surgery and predicting postoperative hearing preservation. Methods: From June 2018 to March 2022, patients with facial spasm and vestibular schwannoma who underwent retrosigmoid approach surgery at the Chinese PLA General Hospital were collected. Before surgery, there were a total of 11 HFS patients and 30 VS patients. Before surgery, the former had hearing level of class A(AAO-HNS), while the latter had serviceable hearing (AAO-HNS class A-B), with a maximum tumor diameter of≤20 mm. CNAP combined with Auditory Brainstem Response (ABR) monitoring was performed during surgery. SPSS 23.0 software was used to analyze the differences of ABR and CNAP parameters between the two groups. Results: The preoperative ABR test in the VS group showed that the latency of wave V and I-V interval were significantly prolonged compared to the HFS group (t=8.36, P<0.001; t=4.61, P<0.001).In VS group, all tumors were totally removed with preserved facial nerve function (House-Brackmann grade Ⅰ-Ⅱ). The serviceable hearing preservation rate was 63.3%. In the HFS group, the initial CNAP waveforms were triphasic and biphasic, with N1 wave latency of (4.32±0.37) ms and amplitude of 25.20 [15.63, 35.40]μV. The distribution of CNAP waveforms before tumor resection in the VS group was the same as that in the HFS group, and which in the hearing preservation group after surgery was also the same. Compared with the HFS group, the latency of N1 wave was prolonged (t=2.670, P=0.011;t=4.584, P<0.001), and the amplitude of N1 wavein the VS group before tumor resection was lower (Z=-3.001, P=0.003). The amplitude of N1 wave in the hearing preservation group after surgery was 20.20 [6.23, 40.90] μV,which was significantly higher than that before tumor resection (Z=-2.133, P=0.033), but there was no statistically significant difference compared to the HFS group (Z=-0.495, P=0.621). Conclusions: The reference range of CNAP in normal hearing population can be preliminarily obtained by the analysis of CNAP in MVD surgery. The latency of N1 wave in VS patients is prolonged and the amplitude of N1 wave is decreased due to tumor compression. The CNAP waveform combined with the change of N1 wave amplitude can provide reference for intraoperative mapping of the cochlear nerve and prediction of postoperative hearing preservation.

From the outer ear to the nerve: A complete computer model of the peripheral auditory system

Computer models of the individual components of the peripheral auditory system - the outer, middle, and inner ears and the auditory nerve - have been developed in the past, with varying level of detail, breadth, and faithfulness of the underlying parameters. Building on previous work, we advance the modeling of the ear by presenting a complete, physiologically justified, bottom-up computer model based on up-to-date experimental data that integrates all of these parts together seamlessly. The detailed bottom-up design of the present model allows for the investigation of partial hearing mechanisms and their defects, including genetic, molecular, and microscopic factors. Also, thanks to the completeness of the model, one can study microscopic effects in the context of their implications on hearing as a whole, enabling the correlation with neural recordings and non-invasive psychoacoustic methods. Such a model is instrumental for advancing quantitative understanding of the mechanism of hearing, for investigating various forms of hearing impairment, as well as for devising next generation hearing aids and cochlear implants.

Ultrastructure of noise-induced cochlear synaptopathy

Acoustic overexposure can eliminate synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs), even if hair-cell function recovers. This synaptopathy has been extensively studied by confocal microscopy, however, understanding the nature and sequence of damage requires ultrastructural analysis. Here, we used focused ion-beam scanning electron microscopy to mill, image, segment and reconstruct ANF terminals in mice, 1 day and 1 week after synaptopathic exposure (8-16 kHz, 98 dB SPL). At both survivals, ANF terminals were normal in number, but 62% and 53%, respectively, lacked normal synaptic specializations. Most non-synapsing fibers (57% and 48% at 1 day and 1 week) remained in contact with an IHC and contained healthy-looking organelles. ANFs showed a transient increase in mitochondrial content (51%) and efferent innervation (34%) at 1 day. Fibers maintaining synaptic connections showed hypertrophy of pre-synaptic ribbons at both 1 day and 1 week. Non-synaptic fibers were lower in mitochondrial content and typically on the modiolar side of the IHC, where ANFs with high-thresholds and low spontaneous rates are normally found. Even 1 week post-exposure, many ANF terminals remained in IHC contact despite loss of synaptic specializations, thus, regeneration efforts at early post-exposure times should concentrate on synaptogenesis rather than neurite extension.

The efficacy of a TrkB monoclonal antibody agonist in preserving the auditory nerve in deafened guinea pigs

The auditory nerve typically degenerates following loss of cochlear hair cells or synapses. In the case of hair cell loss neural degeneration hinders restoration of hearing through a cochlear implant, and in the case of synaptopathy suprathreshold hearing is affected, potentially degrading speech perception in noise. It has been established that neurotrophins such as brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) can mitigate auditory nerve degeneration. Several potential BDNF mimetics have also been investigated for neurotrophic effects in the cochlea. A recent in vitro study showed favorable effects of M3, a TrkB monoclonal antibody agonist, when compared with BDNF. In the present study we set out to examine the effect of M3 on auditory nerve preservation in vivo. Thirty-one guinea pigs were bilaterally deafened, and unilaterally treated with a single 3-µl dose of 7 mg/ml, 0.7 mg/ml M3 or vehicle-only by means of a small gelatin sponge two weeks later. During the experiment and analyses the experimenters were blinded to the three treatment groups. Four weeks after treatment, we assessed the treatment effect (1) histologically, by quantifying survival of SGCs and their peripheral processes (PPs); and (2) electrophysiologically, with two different paradigms of electrically evoked compound action potential (eCAP) recordings shown to be indicative of neural health: single-pulse stimulation with varying inter-phase gap (IPG), and pulse-train stimulation with varying inter-pulse interval. We observed a consistent and significant preservative effect of M3 on SGC survival in the lower basal turn (approximately 40% more survival than in the untreated contralateral cochlea), but also in the upper middle and lower apical turn of the cochlea. This effect was similar for the two treatment groups. Survival of PPs showed a trend similar to that of the SGCs, but was only significantly higher for the highest dose of M3. The protective effect of M3 on SGCs was not reflected in any of the eCAP measures: no statistically significant differences were observed between groups in IPG effect nor between the M3 treatment groups and the control group using the pulse-train stimulation paradigm. In short, while a clear effect of M3 was observed on SGC survival, this was not clearly translated into functional preservation.

Histological Correlates of Auditory Nerve Injury from Kainic Acid in the Budgerigar (Melopsittacus undulatus)

PURPOSE

Loss of auditory nerve afferent synapses with cochlear hair cells, called cochlear synaptopathy, is a common pathology in humans caused by aging and noise overexposure. The perceptual consequences of synaptopathy in isolation from other cochlear pathologies are still unclear. Animal models provide an effective approach to resolve uncertainty regarding the physiological and perceptual consequences of auditory nerve loss, because neural lesions can be induced and readily quantified. The budgerigar, a parakeet species, has recently emerged as an animal model for synaptopathy studies based on its capacity for vocal learning and ability to behaviorally discriminate simple and complex sounds with acuity similar to humans. Kainic acid infusions in the budgerigar produce a profound reduction of compound auditory nerve responses, including wave I of the auditory brainstem response, without impacting physiological hair cell measures. These results suggest selective auditory nerve damage. However, histological correlates of neural injury from kainic acid are still lacking.

METHODS

We quantified the histological effects caused by intracochlear infusion of kainic acid (1 mM; 2.5 µL), and evaluated correlations between the histological and physiological assessments of auditory nerve status.

RESULTS

Kainic acid infusion in budgerigars produced pronounced loss of neural auditory nerve soma (60% on average) in the cochlear ganglion, and of peripheral axons, at time points 2 or more months following injury. The hair cell epithelium was unaffected by kainic acid. Neural loss was significantly correlated with reduction of compound auditory nerve responses and auditory brainstem response wave I.

CONCLUSION

Compound auditory nerve responses and wave I provide a useful index of cochlear synaptopathy in this animal model.

Incidence of Vestibular Schwannoma in Patients with Unilateral Tinnitus: A Systematic Review and Meta-Analysis

OBJECTIVES

Vestibular schwannoma (VS) is a tumor of the vestibulocochlear nerve. Current literature indicates that 1.6% of patients undergoing magnetic resonance imaging of the internal auditory meatus (MRI IAM) for audiovestibular symptoms are diagnosed with a VS. However, there is limited research reporting on patients who present with unilateral tinnitus without asymmetrical hearing loss. This study is a systematic review and meta-analysis evaluating how many of those patients had a VS diagnosed on MRI IAM.

DATABASES USED

Online searches of PubMed, Medline, and Embase databases were performed up to October 2022.

METHODS

This meta-analysis was undertaken aligning with PRISMA guidelines. Articles reporting on patients having MRI IAM for unilateral tinnitus without asymmetrical hearing loss were included. Outcomes measures were patient demographics, VS cases, incidental findings, size, and management of tumor. A meta-analysis of proportions was performed using a random-effects model with the restricted maximum likelihood method. Quality assessment was performed using the Joanna Briggs Institute critical appraisal checklist.

RESULTS

Seven case series were included in the review: a total of 1,394 patients. Seven patients had a VS, with a median size of 4 mm. The pooled detection rate for VS was 0.08% (95% confidence interval = 0.00-0.45). Subsequent management was reported in six cases of which four were actively monitored and two surgically excised. The most common incidental finding was sinus disease (49 patients).

CONCLUSION

Our findings indicate that MRI IAM has a low diagnostic yield for VS detection in patients presenting with unilateral tinnitus without asymmetrical hearing loss, with mostly small tumors that are conservatively managed.

Representation of frequency-modulated sweeps in the cochlear nucleus of the big brown bat

The cochlear nucleus (CN) receives ipsilateral input from the auditory nerve and projects to other auditory brainstem nuclei. Little is known about CN processing of signals used for echolocation. This study recorded multiple unit activity in the CN of anesthetized big brown bats (Eptesicus fuscus) to ultrasonic frequency-modulated (FM) sweeps differing in sweep direction. FM up-sweeps evoke larger peak amplitudes at shorter onset latencies and with smaller amplitude-latency trading ratios than FM down-sweeps. Variability of onset latencies is in the tens of microsecond ranges, indicating sharp temporal precision in the CN for coding of FM signals.

Characteristics of the Summating Potential Measured Across a Cochlear Implant Array as an Indicator of Cochlear Function

OBJECTIVES

The underlying state of cochlear and neural tissue function is known to affect postoperative speech perception following cochlear implantation. The ability to assess these tissues in patients can be performed using intracochlear electrocochleography (IC ECochG). One component of ECochG is the summating potential (SP) that appears to be generated by multiple cochlear tissues. Its qualities may be able to detect the presence of functional inner hair cells, but evidence for this is limited in human cochleae. This study aimed to examine the IC SP characteristics in cochlear implantation recipients, its relationship to preoperative speech perception and audiometric thresholds, and to other IC ECochG components.

DESIGN

This is a retrospective analysis of 113 patients' IC ECochG recordings across the array in response to a 500 Hz tone burst stimulus. Responses to condensation and rarefaction stimuli were then subtracted from one another to emphasize the cochlear microphonic and added to one another to emphasize the SP, auditory nerve neurophonic, and compound action potential. Patients were grouped based on their maximum SP deflection being large and positive (+SP), large and negative (-SP), or minimal (0 SP) to further investigate these relationships.

RESULTS

Patients in the +SP group had better preoperative speech perception (mean consonant-vowel-consonant phoneme score 46%) compared to the -SP and 0 SP groups (consonant-vowel-consonant phoneme scores 34% and 36%, respectively, difference to +SP: p < 0.05). Audiometric thresholds were lowest for +SP (mean pure-tone average 50 dB HL), then -SP (65 dB HL), and highest for 0 SP patients (70 dB HL), but there was not a statistical significance between +SP and -SP groups ( p > 0.1). There were also distinct differences between SP groups in the qualities of their other ECochG components. These included the +SP patients having larger cochlear microphonic maximum amplitude, more apical SP peak electrode locations, and a more spatially specific SP magnitude growth pattern across the array.

CONCLUSIONS

Patients with large positive SP deflection in IC ECochG have preoperatively better speech perception and lower audiometric thresholds than those without. Patterns in other ECochG components suggest its positive deflection may be an indicator of cochlear function.

Relationships Between the Auditory Nerve's Ability to Recover From Neural Adaptation, Cortical Encoding of and Perceptual Sensitivity to Within-channel Temporal Gaps in Postlingually Deafened Adult Cochlear Implant Users

OBJECTIVE

As a step toward identifying the origin of the across-electrode variation in within-channel gap detection thresholds (GDTs) measured in individual cochlear implant (CI) users, this study assessed the relationships between the auditory nerve's (AN's) ability to recover from neural adaptation, cortical encoding of and perceptual sensitivity to within-channel temporal gaps in postlingually deafened adult CI users.

DESIGN

Study participants included 11 postlingually deafened adults with Cochlear Nucleus devices, including three bilaterally implanted participants. In each of the 14 ears tested, recovery from neural adaptation of the AN was measured using electrophysiological measures of the electrically evoked compound action potential at up to four electrode locations. The two CI electrodes in each ear showing the largest difference in the speed of adaptation recovery were selected for assessing within-channel temporal GDT. GDTs were measured using both psychophysical and electrophysiological procedures. Psychophysical GDTs were evaluated using a three-alternative, forced-choice procedure, targeting 79.4% correct on the psychometric function. Electrophysiological GDTs were measured using the electrically evoked auditory event-related potential (eERP) evoked by temporal gaps embedded in electrical pulse trains (i.e., the gap-eERP). Objective GDT was defined as the shortest temporal gap that could evoke a gap-eERP. Related-samples Wilcoxon Signed Rank test was used to compare psychophysical GDTs and objective GDTs measured at all CI electrode locations. It was also used to compare psychophysical GDTs and objective GDTs measured at the two CI electrode locations with different speeds or amounts of adaptation recovery of the AN. A Kendall Rank correlation test was used to assess the correlation between GDTs measured at the same CI electrode location using psychophysical or electrophysiological procedures.

RESULTS

Objective GDTs were significantly larger than those measured using psychophysical procedures. There was a significant correlation between objective and psychophysical GDTs. GDTs could not be predicted based on the amount or the speed of adaptation recovery of the AN.

CONCLUSIONS

Electrophysiological measures of the eERP evoked by temporal gaps can potentially be used to assess within-channel GDT in CI users who cannot provide reliable behavioral responses. The difference in adaptation recovery of the AN is not the primary factor accounting for the across-electrode variation in GDT in individual CI users.

Robust neural tracking of linguistic speech representations using a convolutional neural network

Objective.When listening to continuous speech, populations of neurons in the brain track different features of the signal. Neural tracking can be measured by relating the electroencephalography (EEG) and the speech signal. Recent studies have shown a significant contribution of linguistic features over acoustic neural tracking using linear models. However, linear models cannot model the nonlinear dynamics of the brain. To overcome this, we use a convolutional neural network (CNN) that relates EEG to linguistic features using phoneme or word onsets as a control and has the capacity to model non-linear relations.Approach.We integrate phoneme- and word-based linguistic features (phoneme surprisal, cohort entropy (CE), word surprisal (WS) and word frequency (WF)) in our nonlinear CNN model and investigate if they carry additional information on top of lexical features (phoneme and word onsets). We then compare the performance of our nonlinear CNN with that of a linear encoder and a linearized CNN.Main results.For the non-linear CNN, we found a significant contribution of CE over phoneme onsets and of WS and WF over word onsets. Moreover, the non-linear CNN outperformed the linear baselines.Significance.Measuring coding of linguistic features in the brain is important for auditory neuroscience research and applications that involve objectively measuring speech understanding. With linear models, this is measurable, but the effects are very small. The proposed non-linear CNN model yields larger differences between linguistic and lexical models and, therefore, could show effects that would otherwise be unmeasurable and may, in the future, lead to improved within-subject measures and shorter recordings.

Outcome of cochlear implantation in a case of cochlear aplasia with cochlear nerve deficiency

Cochlear nerve deficiency in cochlear aplasia is a contraindication for cochlear implantation (CI) anticipating poor auditory response. Few authors have reported auditory outcomes even without nerves following CI. This study outlines the audiological outcomes of a patient in early childhood with cochlear aplasia and cochlear nerve deficiency who underwent CI. Auditory and speech-language development were assessed using the Categories of Auditory Performance (CAP) scale, Speech Intelligibility Rating (SIR) scale and Ling-6 sound test; at the time of switch-on, after 6 and 12 months of auditory verbal therapy. Significant differences across CAP, SIR and Ling-6 sound detection scores were noted, with the highest mean scores at the 12th-month postimplantation, indicating substantial improvement in auditory and speech-language skills. In cochlear aplasia cases, residual cochlear elements and nerve fibres cannot be ruled out. Our report emphasises the need for research, as this has the potential to impact the existing guidelines for CI candidacy.

Effects of sensorineural hearing loss on formant-frequency discrimination: Measurements and models

This study concerns the effect of hearing loss on discrimination of formant frequencies in vowels. In the response of the healthy ear to a harmonic sound, auditory-nerve (AN) rate functions fluctuate at the fundamental frequency, F0. Responses of inner-hair-cells (IHCs) tuned near spectral peaks are captured (or dominated) by a single harmonic, resulting in lower fluctuation depths than responses of IHCs tuned between spectral peaks. Therefore, the depth of neural fluctuations (NFs) varies along the tonotopic axis and encodes spectral peaks, including formant frequencies of vowels. This NF code is robust across a wide range of sound levels and in background noise. The NF profile is converted into a rate-place representation in the auditory midbrain, wherein neurons are sensitive to low-frequency fluctuations. The NF code is vulnerable to sensorineural hearing loss (SNHL) because capture depends upon saturation of IHCs, and thus the interaction of cochlear gain with IHC transduction. In this study, formant-frequency discrimination limens (DLFFs) were estimated for listeners with normal hearing or mild to moderate SNHL. The F0 was fixed at 100 Hz, and formant peaks were either aligned with harmonic frequencies or placed between harmonics. Formant peak frequencies were 600 and 2000 Hz, in the range of first and second formants of several vowels. The difficulty of the task was varied by changing formant bandwidth to modulate the contrast in the NF profile. Results were compared to predictions from model auditory-nerve and inferior colliculus (IC) neurons, with listeners' audiograms used to individualize the AN model. Correlations between DLFFs, audiometric thresholds near the formant frequencies, age, and scores on the Quick speech-in-noise test are reported. SNHL had a strong effect on DLFF for the second formant frequency (F2), but relatively small effect on DLFF for the first formant (F1). The IC model appropriately predicted substantial threshold elevations for changes in F2 as a function of SNHL and little effect of SNHL on thresholds for changes in F1.

Central projections of auditory nerve fibers in the western rat snake (Pantherophis obsoletus)

Despite the absence of tympanic middle ears, snakes can hear. They are thought to primarily detect substrate vibration via connections between the lower jaw and the inner ear. We used the western rat snake (Pantherophis obsoletus) to determine how vibration is processed in the brain. We measured vibration-evoked potential recordings to reveal sensitivity to low-frequency vibrations. We then used tract tracing combined with immunohistochemistry and Nissl staining to describe the central projections of the papillar branch of the VIIIth nerve. Applications of biotinylated dextran amine to the basilar papilla (homologous to the organ of Corti of mammals) labeled bouton-like terminals in two first-order cochlear nuclei, a rostrolateral nucleus angularis (NA) and a caudomedial nucleus magnocellularis (NM). NA formed a distinct dorsal eminence, consisted of heterogenous cell types, and was parvalbumin positive. NM was smaller and poorly separated from the surrounding vestibular nuclei. NM was distinguished by positive calbindin label and included fusiform and round cells. Thus, the atympanate western rat snake shares similar first-order projections to tympanate reptiles. Auditory pathways may be used for detecting vibration, not only in snakes but also potentially in atympanate early tetrapods.

Whole Neonatal Cochlear Explants as an In vitro Model

Untreated hearing loss imposes significant costs on the global healthcare system and impairs individuals' quality of life. Sensorineural hearing loss is characterized by the cumulative and irreversible loss of sensory hair cells and auditory nerves in the cochlea. Entire and vital cochlear explants are one of the fundamental tools in hearing research to detect hair cell loss and to characterize the molecular mechanisms of the inner ear cells. Many years ago, a protocol for neonatal cochlear isolation was developed, and although it has been modified over time, it still holds potential for improvement. This paper presents an optimized protocol for isolating and culturing whole neonatal cochlear explants in multi-well culture chambers that enables the study of hair cells and spiral ganglion neuron cells along the entire length of the cochlea. The protocol was tested using cochlear explants from mice and rats. Healthy cochlear explants were obtained to study the interaction between hair cells, spiral ganglion neuron cells, and the surrounding supporting cells. One of the main advantages of this method is that it simplifies the organ culture steps without compromising the quality of the explants. All three turns of the organ of Corti are attached to the bottom of the chamber, which facilitates in vitro experiments and the comprehensive analysis of the explants. We provide some examples of cochlear images from different experiments with live and fixed explants, demonstrating that the explants retain their structure despite exposure to ototoxic drugs. This optimized protocol can be widely used for the integrative analysis of the mammalian cochlea.

Computational modeling of the human compound action potential

The auditory nerve (AN) compound action potential (CAP) is an important tool for assessing auditory disorders and monitoring the health of the auditory periphery during surgical procedures. The CAP has been mathematically conceptualized as the convolution of a unit response (UR) waveform with the firing rate of a population of AN fibers. Here, an approach for predicting experimentally recorded CAPs in humans is proposed, which involves the use of human-based computational models to simulate AN activity. CAPs elicited by clicks, chirps, and amplitude-modulated carriers were simulated and compared with empirically recorded CAPs from human subjects. In addition, narrowband CAPs derived from noise-masked clicks and tone bursts were simulated. Many morphological, temporal, and spectral aspects of human CAPs were captured by the simulations for all stimuli tested. These findings support the use of model simulations of the human CAP to refine existing human-based models of the auditory periphery, aid in the design and analysis of auditory experiments, and predict the effects of hearing loss, synaptopathy, and other auditory disorders on the human CAP.

Dendritic Degeneration and Altered Synaptic Innervation of a Central Auditory Neuron During Age-related Hearing Loss

Cellular morphology and synaptic configuration are key determinants of neuronal function and are often modified under pathological conditions. In the first nucleus of the central auditory system, the cochlear nucleus (CN), principal bushy neurons specialize in processing temporal information of sound critical for hearing. These neurons alter their physiological properties during aging that contribute to age-related hearing loss (ARHL). The structural basis of such changes remains unclear, especially age-related modifications in their dendritic morphology and the innervating auditory nerve (AN) synapses. Using young (2-5 months) and aged (28-33 months) CBA/CaJ mice of either sex, we filled individual bushy neurons with fluorescent dye in acute brain slices to characterize their dendritic morphology, followed by immunostaining against vesicular glutamate transporter 1 (VGluT1) and calretinin (CR) to identify innervating AN synapses. We found that dendritic morphology of aged bushy neurons had significantly reduced complexity, suggesting age-dependent dendritic degeneration, especially in neurons with predominantly non-CR-expressing synapses on the soma. These dendrites were innervated by AN bouton synapses, which were predominantly non-CR-expressing in young mice but had increased proportion of CR-expressing synapses in old mice. While somatic AN synapses degenerated substantially with age, as quantified by VGluT1-labeled puncta volume, no significant difference was observed in the total volume of dendritic synapses between young and old mice. Consequently, synaptic density on dendrites was significantly higher in old mice. The findings suggest that dendritic degeneration and altered synaptic innervation in bushy neurons during aging may underlie their changed physiological activity and contribute to the development of ARHL.

Relationships Between the Auditory Nerve Sensitivity to Amplitude Modulation, Perceptual Amplitude Modulation Rate Discrimination Sensitivity, and Speech Perception Performance in Postlingually Deafened Adult Cochlear Implant Users

OBJECTIVE

This study assessed the relationships between the salience of amplitude modulation (AM) cues encoded at the auditory nerve (AN), perceptual sensitivity to changes in AM rate (i.e., AM rate discrimination threshold, AMRDT), and speech perception scores in postlingually deafened adult cochlear implant (CI) users.

DESIGN

Study participants were 18 postlingually deafened adults with Cochlear Nucleus devices, including five bilaterally implanted patients. For each of 23 implanted ears, neural encoding of AM cues at 20 Hz at the AN was evaluated at seven electrode locations across the electrode array using electrophysiological measures of the electrically evoked compound action potential (eCAP). The salience of AM neural encoding was quantified by the Modulated Response Amplitude Ratio (MRAR). Psychophysical measures of AMRDT for 20 Hz modulation were evaluated in 16 ears using a three-alternative, forced-choice procedure, targeting 79.4% correct on the psychometric function. AMRDT was measured at up to five electrode locations for each test ear, including the electrode pair that showed the largest difference in the MRAR. Consonant-Nucleus-Consonant (CNC) word scores presented in quiet and in speech-shaped noise at a signal to noise ratio (SNR) of +10 dB were measured in all 23 implanted ears. Simulation tests were used to assess the variations in correlation results when using the MRAR and AMRDT measured at only one electrode location in each participant to correlate with CNC word scores. Linear Mixed Models (LMMs) were used to evaluate the relationship between MRARs/AMRDTs measured at individual electrode locations and CNC word scores. Spearman Rank correlation tests were used to evaluate the strength of association between CNC word scores measured in quiet and in noise with (1) the variances in MRARs and AMRDTs, and (2) the averaged MRAR or AMRDT across multiple electrodes tested for each participant.

RESULTS

There was no association between the MRAR and AMRDT. Using the MRAR and AMRDT measured at only one, randomly selected electrode location to assess their associations with CNC word scores could lead to opposite conclusions. Both the results of LMMs and Spearman Rank correlation tests showed that CNC word scores measured in quiet or at 10 dB SNR were not significantly correlated with the MRAR or AMRDT. In addition, the results of Spearman Rank correlation tests showed that the variances in MRARs and AMRDTs were not significantly correlated with CNC word scores measured in quiet or in noise.

CONCLUSIONS

The difference in AN sensitivity to AM cues is not the primary factor accounting for the variation in AMRDTs measured at different stimulation sites within individual CI users. The AN sensitivity to AM per se may not be a crucial factor for CNC word perception in quiet or at 10 dB SNR in postlingually deafened adult CI users. Using electrophysiological or psychophysical results measured at only one electrode location to correlate with speech perception scores in CI users can lead to inaccurate, if not wrong, conclusions.

Speech Perception Performance in Cochlear Implant Recipients Correlates to the Number and Synchrony of Excited Auditory Nerve Fibers Derived From Electrically Evoked Compound Action Potentials

OBJECTIVES

Many studies have assessed the performance of individuals with cochlear implants (CIs) with electrically evoked compound action potentials (eCAPs). These eCAP-based studies have focused on the amplitude information of the response, without considering the temporal firing properties of the excited auditory nerve fibers (ANFs), such as neural latency and synchrony. These temporal features have been associated with neural health in animal studies and, consequently, could be of importance to clinical CI outcomes. With a deconvolution method, combined with a unitary response, the eCAP can be mathematically unraveled into the compound discharge latency distribution (CDLD). The CDLD reflects both the number and the temporal firing properties of excited ANFs. The present study aimed to determine to what extent the CDLD derived from intraoperatively recorded eCAPs is related to speech perception in individuals with CIs.

DESIGN

This retrospective study acquired data on monosyllabic word recognition scores and intraoperative eCAP amplitude growth functions from 124 adult patients with postlingual deafness that received the Advanced Bionics HiRes 90K device. The CDLD was determined for each recorded eCAP waveform by deconvolution. Each of the two Gaussian components of the CDLD was described by three parameters: the amplitude, the firing latency (the average latency of each component of the CDLD), and the variance of the CDLD components (an indication of the synchronicity of excited ANFs). Apart from these six CDLD parameters, the area under the CDLD curve (AUCD) and the slope of the AUCD growth function were determined as well. The AUCD was indicative of the total number of excited ANFs over time. The slope of the AUCD growth function indicated the increases in the number of excited ANFs with stimulus level. Associations between speech perception and each of these eight CDLD-related parameters were investigated with linear mixed modeling.

RESULTS

In individuals with CIs, larger amplitudes of the two CDLD components, greater AUCD, and steeper slopes of the AUCD growth function were all significantly associated with better speech perception. In addition, a smaller latency variance in the early CDLD component, but not in the late, was significantly associated with better speech recognition scores. Speech recognition was not significantly dependent on CDLD latencies. The AUCD and the slope of the AUCD growth function provided a similar explanation of the variance in speech perception (R 2 ) as the eCAP amplitude, the slope of the amplitude growth function, the amplitude, and variance of the first CDLD component.

CONCLUSION

The results demonstrate that both the number and the neural synchrony of excited ANFs, as revealed by CDLDs, are indicative of postimplantation speech perception in individuals with a CI. Because the CDLD-based parameters yielded a higher significance than the eCAP amplitude or the AGF slope, the authors conclude that CDLDs can serve as a clinical predictor of the survival of ANFs and that they have predictive value for postoperative speech perception performance. Thus, it would be worthwhile to incorporate the CDLD into eCAP measures in future clinical applications.

Intra-cochlear differences in the spread of excitation between biphasic and triphasic pulse stimulation in cochlear implants: A modeling and experimental study

Triphasic pulse stimulation can prevent unpleasant facial nerve stimulation in cochlear implant users. Using electromyographic measurements on facial nerve effector muscles, previous studies have shown that biphasic and triphasic pulse stimulations produce different input-output functions. However, little is known about the intracochlear effects of triphasic stimulation and how these may contribute to the amelioration of facial nerve stimulation. The present study used a computational model of implanted human cochleae to investigate the effect of pulse shape on the intracochlear spread of excitation. Biphasic and triphasic pulse stimulations were simulated from three different cochlear implant electrode contact positions. To validate the model results, experimental spread of excitation measurements were conducted with biphasic and triphasic pulse stimulation from three different electrode contact positions in 13 cochlear implant users. The model results depict differences between biphasic and triphasic pulse stimulations depending on the position of the stimulating electrode contact. While biphasic and triphasic pulse stimulations from a medial or basal electrode contact caused similar extents of neural excitation, differences between the pulse shapes were observed when the stimulating contact was located in the cochlear apex. In contrast, the experimental results showed no difference between the biphasic and triphasic initiated spread of excitation for any of the tested contact positions. The model was also used to study responses of neurons without peripheral processes to mimic the effect of neural degeneration. For all three contact positions, simulated degeneration shifted the neural responses towards the apex. Biphasic pulse stimulation showed a stronger response with neural degeneration compared to without degeneration, while triphasic pulse stimulation showed no difference. As previous measurements have demonstrated an ameliorative effect of triphasic pulse stimulation on facial nerve stimulation from medial electrode contact positions, the results imply that a complementary effect located at the facial nerve level must be responsible for reducing facial nerve stimulation.

Isolating auditory-nerve contributions to electrocochleography by high-pass filtering: A better biomarker for cochlear nerve degeneration?

In search of biomarkers for cochlear neural degeneration (CND) in electrocochleography from humans with normal thresholds, we high-pass and low-pass filtered the responses to separate contributions of auditory-nerve action potentials (N1) from hair-cell summating potentials (SP). The new N1 measure is better correlated with performance on difficult word-recognition tasks used as a proxy for CND. Furthermore, the paradoxical correlation between larger SPs and worse word scores, observed with classic electrocochleographic analysis, disappears with the new metric. Classic SP is simultaneous with and opposite in phase to an early neural contribution, and filtering separates the sources to eliminate this interference.

[Prospects for cochlear implantation in patients with auditory nerve dysplasia]

Auditory nerve dysplasia (AND) can encompass various conditions of the auditory nerve (AN), ranging from true aplasia to hypoplasia. The purpose of this review is to discuss the prospect of cochlear implantation (CI) and subsequent auditory speech rehabilitation for AN abnormality. Studies of different authors when working with this category of children, possible results and methods of diagnostics of the AN condition are presented.

Contextual Lateralization Based on Interaural Level Differences Is Preshaped by the Auditory Periphery and Predominantly Immune Against Sequential Segregation

The perceived azimuth of a target sound is determined by the interaural time difference and the interaural level difference (ILD) and is subject to contextual effects from precursor sounds. This study characterized ILD-based precursor effects (PEs) for high-frequency stimuli in a total of seven normal-hearing listeners. In Experiment 1, precursor and target were band-pass-filtered noises approximately centered at 4 kHz (1.2- and 1-octave bandwidth, respectively) separated by a 10-ms gap. The effects of precursor location (ipsilateral, contralateral, and central) on the perceived target azimuth were measured using a head-pointing task. Relative to control trials without a precursor, ipsilateral precursors biased the perceived target azimuth toward midline (medial bias) and contralateral precursors biased it contralaterally (lateral bias). Central precursors caused a symmetric lateral bias. An auditory periphery model that determines the "internal" ILD at the auditory nerve level, including either realistic efferent compression control or auditory nerve adaptation, explained about 50% of the variance in the PEs. These within-trial PEs were accompanied by an across-trial PE, inducing medial bias. Experiment 2 studied the role of sequential segregation in the within-trial PE by introducing a pitch difference between precursor and target. Segregation conditions caused increased PE for ipsilateral, no effect for contralateral, and either no effect or reduced PE for central precursors. Overall, the ILD-based within-trial PE appears to be preshaped already in the auditory periphery and the mechanism underlying at least the ipsilateral PE appears to be immune against sequential segregation.

Temporal Envelope Coding of the Human Auditory Nerve Inferred from Electrocochleography: Comparison with Envelope Following Responses

Neural coding of the slow amplitude fluctuations of sound (i.e., temporal envelope) is thought to be essential for speech understanding; however, such coding by the human auditory nerve is poorly understood. Here, neural coding of the temporal envelope by the human auditory nerve is inferred from measurements of the compound action potential in response to an amplitude modulated carrier (CAPENV) for modulation frequencies ranging from 20 to 1000 Hz. The envelope following response (EFR) was measured simultaneously with CAPENV from active electrodes placed on the high forehead and tympanic membrane, respectively. Results support the hypothesis that phase locking to higher modulation frequencies (> 80 Hz) will be stronger for CAPENV, compared to EFR, consistent with the upper-frequency limits of phase locking for auditory nerve fibers compared to auditory brainstem/cortex neurons. Future work is needed to determine the extent to which (1) CAPENV is a useful tool for studying how temporal processing of the auditory nerve is affected by aging, hearing loss, and noise-induced cochlear synaptopathy and (2) CAPENV reveals the relationship between auditory nerve temporal processing and perception of the temporal envelope.

Expression and Neurotransmitter Association of the Synaptic Calcium Sensor Synaptotagmin in the Avian Auditory Brain Stem

In the avian auditory brain stem, acoustic timing and intensity cues are processed in separate, parallel pathways via the two divisions of the cochlear nucleus, nucleus angularis (NA) and nucleus magnocellularis (NM). Differences in excitatory and inhibitory synaptic properties, such as release probability and short-term plasticity, contribute to differential processing of the auditory nerve inputs. We investigated the distribution of synaptotagmin, a putative calcium sensor for exocytosis, via immunohistochemistry and double immunofluorescence in the embryonic and hatchling chick brain stem (Gallus gallus). We found that the two major isoforms, synaptotagmin 1 (Syt1) and synaptotagmin 2 (Syt2), showed differential expression. In the NM, anti-Syt2 label was strong and resembled the endbulb terminals of the auditory nerve inputs, while anti-Syt1 label was weaker and more punctate. In NA, both isoforms were intensely expressed throughout the neuropil. A third isoform, synaptotagmin 7 (Syt7), was largely absent from the cochlear nuclei. In nucleus laminaris (NL, the target nucleus of NM), anti-Syt2 and anti-Syt7 strongly labeled the dendritic lamina. These patterns were established by embryonic day 18 and persisted to postnatal day 7. Double-labeling immunofluorescence showed that Syt1 and Syt2 were associated with vesicular glutamate transporter 2 (VGluT2), but not vesicular GABA transporter (VGAT), suggesting that these Syt isoforms were localized to excitatory, but not inhibitory, terminals. These results suggest that Syt2 is the major calcium binding protein underlying excitatory neurotransmission in the timing pathway comprising NM and NL, while Syt2 and Syt1 regulate excitatory transmission in the parallel intensity pathway via cochlear nucleus NA.

The Effects of Middle-ear Stiffness on the Auditory Brainstem Neural Encoding of Phase

The middle-ear system relies on a balance of mass and stiffness characteristics for transmitting sound from the external environment to the cochlea and auditory neural pathway. Phase is one aspect of sound that, when transmitted and encoded by both ears, contributes to binaural cue sensitivity and spatial hearing. The study aims were (i) to investigate the effects of middle-ear stiffness on the auditory brainstem neural encoding of phase in human adults with normal pure-tone thresholds and (ii) to investigate the relationships between middle-ear stiffness-induced changes in wideband acoustic immittance and neural encoding of phase. The auditory brainstem neural encoding of phase was measured using the auditory steady-state response (ASSR) with and without middle-ear stiffness elicited via contralateral activation of the middle-ear muscle reflex (MEMR). Middle-ear stiffness was quantified using a wideband acoustic immittance assay of acoustic absorbance. Statistical analyses demonstrated decreased ASSR phase lag and decreased acoustic absorbance with contralateral activation of the MEMR, consistent with increased middle-ear stiffness changing the auditory brainstem neural encoding of phase. There were no statistically significant correlations between stiffness-induced changes in wideband acoustic absorbance and ASSR phase. The findings of this study may have important implications for understanding binaural cue sensitivity and horizontal plane sound localization in audiologic and otologic clinical populations that demonstrate changes in middle-ear stiffness, including cochlear implant recipients who use combined electric and binaural acoustic hearing and otosclerosis patients.

A Computational Model of a Single Auditory Nerve Fiber for Electric-Acoustic Stimulation

Cochlear implant (CI) recipients with preserved acoustic low-frequency hearing in the implanted ear are a growing group among traditional CI users who benefit from hybrid electric-acoustic stimulation (EAS). However, combined ipsilateral electric and acoustic stimulation also introduces interactions between the two modalities that can affect the performance of EAS users. A computational model of a single auditory nerve fiber that is excited by EAS was developed to study the interaction between electric and acoustic stimulation. Two existing models of sole electric or acoustic stimulation were coupled to simulate responses to combined EAS. Different methods of combining both models were implemented. In the coupled model variant, the refractoriness of the simulated fiber leads to suppressive interaction between electrically evoked and acoustically evoked spikes as well as spontaneous activity. The second model variant is an uncoupled EAS model without electric-acoustic interaction. By comparing predictions between the coupled and the noninteracting EAS model, it was possible to infer electric-acoustic interaction at the level of the auditory nerve. The EAS model was used to simulate fiber populations with realistic inter-unit variability, where each unit was represented by the single-fiber model. Predicted thresholds and dynamic ranges, spike rates, latencies, jitter, and vector strengths were compared to empirical data. The presented EAS model provides a framework for future studies of peripheral electric-acoustic interaction.

Postlingually Deafened Adult Cochlear Implant Users With Prolonged Recovery From Neural Adaptation at the Level of the Auditory Nerve Tend to Have Poorer Speech Perception Performance

OBJECTIVE

This study investigated the effects of two temporal response properties of the auditory nerve (i.e., neural adaptation and recovery from neural adaptation) on speech perception performance in postlingually deafened adult cochlear implant (CI) users.

DESIGN

Study participants included 18 postlingually deafened adults who were Cochlear Nucleus device users with a full electrode array insertion in the test ear(s). Neural adaptation and adaptation recovery of the auditory nerve (AN) were evaluated using electrophysiological measures of the electrically evoked compound action potential (eCAP). The amount of neural adaptation was quantified by the adaptation index within three time windows: 0 to 8.89 (window 1), 44.44 to 50.00 (window 2), and 94.44 to 100.00 ms (window 3). The speed of neural adaptation was estimated using a two-parameter power law function. To evaluate adaptation recovery of the AN, eCAPs to the last pulse of the 100-ms pulse train were recorded at masker-probe-intervals ranging from 1.054 to 256 ms in logarithmic steps. The amount of adaptation recovery was quantified by the adaptation recovery ratio. The time-constant of adaptation recovery was estimated using an exponential function with up to three components. Speech perception performance was evaluated by measuring consonant-nucleus-consonant (CNC) word scores presented in quiet and in speech-shaped noise at a signal-to-noise ratio (SNR) of +10 dB. One-tailed Pearson Product Moment correlation tests were used (1) to assess the associations among parameters of neural adaptation and adaptation recovery and (2) to evaluate the strength of association between these parameters and CNC word scores measured in quiet and in noise. The contributions of different parameters quantifying neural adaptation and adaptation recovery on speech perception scores were evaluated using multivariable linear regression analyses.

RESULTS

The Pearson Product Moment correlation coefficient demonstrated a moderate, negative correlation between the speed of adaptation recovery and CNC word scores measured in quiet and in noise. The speed of adaptation recovery accounted for 14.1% of variability in CNC word scores measured in quiet and 16.7% of variability in CNC word scores measured in noise. The correlation strengths between CNC word scores and the adaptation index, the adaptation recovery ratio and the speed of neural adaptation ranged from negligible to weak.

CONCLUSIONS

The speed of adaptation recovery plays a more important role than other features of neural adaptation and adaptation recovery of the AN in speech perception in postlingually deafened adult CI users. Patients with prolonged adaptation recovery tend to show poorer speech perception performance.

Peripheral Auditory Nerve Impairment in a Mouse Model of Syndromic Autism

Dysfunction of the peripheral auditory nerve (AN) contributes to dynamic changes throughout the central auditory system, resulting in abnormal auditory processing, including hypersensitivity. Altered sound sensitivity is frequently observed in autism spectrum disorder (ASD), suggesting that AN deficits and changes in auditory information processing may contribute to ASD-associated symptoms, including social communication deficits and hyperacusis. The MEF2C transcription factor is associated with risk for several neurodevelopmental disorders, and mutations or deletions of MEF2C produce a haploinsufficiency syndrome characterized by ASD, language, and cognitive deficits. A mouse model of this syndromic ASD (Mef2c-Het) recapitulates many of the MEF2C haploinsufficiency syndrome-linked behaviors, including communication deficits. We show here that Mef2c-Het mice of both sexes exhibit functional impairment of the peripheral AN and a modest reduction in hearing sensitivity. We find that MEF2C is expressed during development in multiple AN and cochlear cell types; and in Mef2c-Het mice, we observe multiple cellular and molecular alterations associated with the AN, including abnormal myelination, neuronal degeneration, neuronal mitochondria dysfunction, and increased macrophage activation and cochlear inflammation. These results reveal the importance of MEF2C function in inner ear development and function and the engagement of immune cells and other non-neuronal cells, which suggests that microglia/macrophages and other non-neuronal cells might contribute, directly or indirectly, to AN dysfunction and ASD-related phenotypes. Finally, our study establishes a comprehensive approach for characterizing AN function at the physiological, cellular, and molecular levels in mice, which can be applied to animal models with a wide range of human auditory processing impairments.SIGNIFICANCE STATEMENT This is the first report of peripheral auditory nerve (AN) impairment in a mouse model of human MEF2C haploinsufficiency syndrome that has well-characterized ASD-related behaviors, including communication deficits, hyperactivity, repetitive behavior, and social deficits. We identify multiple underlying cellular, subcellular, and molecular abnormalities that may contribute to peripheral AN impairment. Our findings also highlight the important roles of immune cells (e.g., cochlear macrophages) and other non-neuronal elements (e.g., glial cells and cells in the stria vascularis) in auditory impairment in ASD. The methodological significance of the study is the establishment of a comprehensive approach for evaluating peripheral AN function and impact of peripheral AN deficits with minimal hearing loss.

Combining Place and Rate of Stimulation Improves Frequency Discrimination in Cochlear Implant Users

In the auditory system, frequency is represented as tonotopic and temporal response properties of the auditory nerve. While these response properties are inextricably linked in normal hearing, cochlear implants can separately excite tonotopic location and temporal synchrony using different electrodes and stimulation rates, respectively. This separation allows for the investigation of the contributions of tonotopic and temporal cues for frequency discrimination. The present study examines frequency discrimination in adult cochlear implant users as conveyed by electrode position and stimulation rate, separately and combined. The working hypothesis is that frequency discrimination is better provided by place and rate cues combined compared to either cue alone. This hypothesis was tested in two experiments. In the first experiment, frequency discrimination needed for melodic contour identification was measured for frequencies near 100, 200, and 400 Hz using frequency allocation modeled after clinical processors. In the second experiment, frequency discrimination for pitch ranking was measured for frequencies between 100 and 1600 Hz using an experimental frequency allocation designed to provide better access to place cues. The results of both experiments indicate that frequency discrimination is better with place and rate cues combined than with either cue alone. These results clarify how signal processing for cochlear implants could better encode frequency into place and rate of electrical stimulation. Further, the results provide insight into the contributions of place and rate cues for pitch.

[Cochlear nerve canal stenosis: a review of recent research]

Some patients with severe-profound sensorineural hearing loss （SNHL） with normal cochlear anatomical structure received cochlear implantation （CI） and the hearing and speech rehabilitation effect was not ideal. Through retrospective analysis, it was found that some of these patients had cochlear never canal （CNC） stenosis, or atresia in severe cases.This article reviews the development of the CNC, the diagnostic criteria of CNC stenosis and the results of hearing and speech rehabilitation in these patients after CI.

Concept and first Implementation of an intracochlearly navigated Electrode Array for Cochlear Implantation

Cochlear implants (CI) are an established treatment for people with deafness or severe hearing loss. To restore patients' hearing an electrode array (EA) of the CI is inserted into the cochlea to stimulate the auditory nerve. Thereby, the exact positioning and gentle insertion of the EA is crucial for optimal hearing perception outcome. Currently, only microscopic vision is available for entering the cochlea, but the critical intracochlear process during EA insertion is like a "black box" and the surgeon has to rely on haptic feedback. Methods for visualizing the insertion process during surgery are inaccurate or not suitable for routine use due to radiation exposure. To address this problem, we developed a computer-assisted and image-guided cochlear implantation system with an exact real-time visualization of the EA position during the insertion process. The system is based on an electromagnetic tracking system that measures the position and orientation of a sensor integrated into the tip of a EA prototype and visualizes it in presurgical image data. A first experiment with our system showed that a EA prototype could be inserted into a cochlea of a human temporal bone and placed with an accuracy of [Formula: see text]. A maximum insertion angle of 120° was achieved.

Comparison of Indicators of Short and Long Latency Auditory Evoked Potentials in Newborns

Auditory evoked potentials (EVP) are an effective and non-invasive way to assess the functional state of the auditory nerve and auditory sensory pathways of the brainstem. An increase in the latent periods of peaks III, IV, V, as well as a lengthening of the time of central sound conduction of III-V and I-V waves in all newborns with hyperbilirubinemia, indicates a pathology of hearing of central origin with impaired conduction along the auditory pathways at the level of the lower and middle third of the brain pons.

[Relationship between sudden sensorineural hearing loss and facial auditory nerve and anterior inferior cerebellar artery vascular loop in the internal auditory tract MRI]

Objective:To investigate the correlation between the facial auditory nerve and anterior inferior cerebellar artery vascular loop in MRI of the internal auditory meatus and idiopathic sudden sensorineural hearing loss. Methods:This retrospective study enrolled 144 patients with idiopathic sudden sensorineural hearing loss（SSNHL）（experimental group） and 36 healthy subjects with 72 ears（control group）, who attended the Department of Otolaryngology Head and Neck Surgery, the 940th Hospital of the Joint Logistics Support Unit of the Chinese PLA from January, 2019 to January, 2021. The magnetic resonance imaging（MRI） data of the internal auditory meatus and clinical data were collected and compared between the two groups. Results:The distance between the auditory nerve and the peripheral vessels in the unilateral SSNHL-affected ear was significantly different from that in the contralateral ears and that in the healthy ears of the control group （P<0.05）. The distance between the auditory nerve and the peripheral vessels in both ear affected by bilateral SSNHL was significantly different from that in the healthy ears of the control group （P<0.05）. There was no significant difference in radiological grading of vascular loops between the ears affected by unilateral SSNHL and the contralateral ears and the healthy ear of the control group （P>0.05）. No statistically significant differences in radiological grading of vascular loops were found between both ears with bilateral SSNHL and the healthy ears in the control group（P>0.05）. The severity of hearing loss, audiometric configuration, radiological grading of vascular loops and the distance between the facial auditory nerve and peripheral vessels were not significantly different between the affected ears in unilateral SSNHL and both ears in bilateral SSNHL （P>0.05）. Conclusion:SSNHL is associated with the distance between the auditory nerve and the nearest peripheral vessel. SSNHL may occur when the vessel compresses the auditory nerve.

Brainstem auditory evoked potential combined with high resolution cranial base CT can optimize the diagnosis of auditory nerve injury

PURPOSE

Auditory nerve injury is one of the most common nerve injury complications of skull base fractures. However, there is currently a lack of auxiliary examination methods for its direct diagnosis. The purpose of this study was to find a more efficient and accurate means of diagnosis for auditory nerve injury.

METHODS

Through retrospectively analyzing the results of brainstem auditory evoked potential (BAEP) and high-resolution CT (HRCT) in 37 patients with hearing impairment following trauma from January 1, 2018 to July 31, 2020, the role of the two inspection methods in the diagnosis of auditory nerve injury was studied. Inclusion criteria were patient had a clear history of trauma and unilateral hearing impairment after trauma; while exclusion criteria were: (1) severe patient with a Glasgow coma scale score ≤5 because these patients were classified as severe head injury and admitted to the intensive care unit, (2) patient in the subacute stage admitted 72 h after trauma, and (3) patient with prior hearing impairment before trauma. According to Goodman's classification of hearing impairment, the patients were divided into low/medium/severe injury groups. In addition, patients were divided into HRCT-positive and negative groups for further investigation with their BAEP results. The positive rates of BEAP for each group were observed, and the results were analyzed by Chi-square test (p < 0.05, regarded as statistical difference).

RESULTS

A total of 37 patients were included, including 21 males and 16 females. All of them were hospitalized patients with GCS score of 6-15 at the time of admission. The BAEP positive rate in the medium and severe injury group was 100%, which was significantly higher than that in the low injury group (27.27%) (p < 0.01). The rate of BEAP positivity was significantly higher in the HRCT-positive group (20/30, 66.7%) than in the HRCT-negative group (1/7, 14.3%) (p < 0.05). Twenty patients (54.05%) were both positive for BEAP and HRCT test, and considered to have auditory nerve damage. Six patients (16.22%) were both negative for BEAP and HRCT test, and 10 patients (27.03%) were BAEP-negative but HRCT-positive: all the 16 patients were considered as non-neurological injury. The rest 1 case (2.70%) was BAEP-positive but HRCT-negative, which we speculate may have auditory nerve concussion.

CONCLUSION

By way of BAEP combining with skull base HRCT, we may improve the accuracy of the diagnosis of auditory nerve injury. Such a diagnostic strategy may be beneficial to guiding treatment plans and evaluating prognosis.

Management of Severe Facial Nerve Cross Stimulation by Cochlear Implant Replacement to Change Pulse Shape and Grounding Configuration: A Case-series

OBJECTIVES

To investigate the combined effect of changing pulse shape and grounding configuration to manage facial nerve stimulation (FNS) in cochlear implant (CI) recipients.

PATIENTS

Three adult CI recipients with severe FNS were offered a replacement implant when standard stimulation strategies and programming adjustments did not resolve symptoms. Our hypothesis was that the facial nerve was less likely to be activated when using anodic pulses with "mixed-mode" intra-cochlear and extra-cochlear current return.

INTERVENTION

All patients were reimplanted with an implant that uses a pseudo-monophasic anodic pulse shape, with mixed-mode grounding (stimulus mixed-mode anodic)-the Neuro Zti CI (Oticon Medical). This device also allows measurements of neural function and loudness with monopolar, symmetric biphasic pulses (stimulus MB), the clinical standard used by most CIs as a comparison.

MAIN OUTCOME MEASURES

The combined effect of pulse shape and grounding configuration on FNS was monitored during surgery. Following CI activation, FNS symptoms and performance with the Neuro Zti implant were compared with outcomes before reimplantation.

RESULTS

FNS could only be recorded using stimulus MB for all patients. In clinical use, all patients reported reduced FNS and showed an improvement in Bamford-Kowal-Bench sentences recognition compared with immediately before reimplantation. Bamford-Kowal-Bench scores with a male speaker were lower compared with measurements taken before the onset of severe FNS for patients 1 and 2.

CONCLUSIONS

In patients where CI auditory performance was severely limited by FNS, charge-balanced pseudo-monophasic stimulation mode with a mixed-mode grounding configuration limited FNS and improved loudness percept compared with standard biphasic stimulation with monopolar grounding.

Simulation of ITD-Dependent Single-Neuron Responses Under Electrical Stimulation and with Amplitude-Modulated Acoustic Stimuli

Interaural time difference (ITD) sensitivity with cochlear implant stimulation is remarkably similar to envelope ITD sensitivity using conventional acoustic stimulation. This holds true for human perception, as well as for neural response rates recorded in the inferior colliculus of several mammalian species. We hypothesize that robust excitatory-inhibitory (EI) interaction is the dominant mechanism. Therefore, we connected the same single EI-model neuron to either a model of the normal acoustic auditory periphery or to a model of the electrically stimulated auditory nerve. The model captured most features of the experimentally obtained response properties with electric stimulation, such as the shape of rate-ITD functions, the dependence on stimulation level, and the pulse rate or modulation-frequency dependence. Rate-ITD functions with high-rate, amplitude-modulated electric stimuli were very similar to their acoustic counterparts. Responses obtained with unmodulated electric pulse trains most resembled acoustic filtered clicks. The fairly rapid decline of ITD sensitivity at rates above 300 pulses or cycles per second is correctly simulated by the 3.1-ms time constant of the inhibitory post-synaptic conductance. As the model accounts for these basic properties, it is expected to help in understanding and quantifying the binaural hearing abilities with electric stimulation when integrated in bigger simulation frameworks.

[Perioperative management of cochlear implantation for CHARGE syndrome]

Objective:To explore the perioperative period characteristics of paediatric cochlear implant recipients of CHARGE syndrome with complex deformities. Methods:Retrospective case series of CHARGE syndrome were included. Radiological results, intraoperative findings, surgical planning and post-operative complications were analyzed. Routine audiometric measurements, speech perception categories and speech intelligibility ratings were performed pre and post-operatively to measure auditory speech rehabilitation outcomes. Results:Five prelingual profoundly deaf children were identified, aged from 14 months to 60 months. All patients had congenital heart disease and underwent surgery before cochlear implantation. Upper airway abnormalities were detected as choanal atresia, laryngomalacia and tracheal stenosis. All ten ears showed cochlear abnormalities（Incomplete partition Ⅱ）, eight of them combined with secretory otitis media and/or middle ear deformity. All patients underwent single side surgery using standard transmastoid facial recess approach. Full insertion of the electrode was achieved in two cochleas, while partial insertion was done in three cochleas. Three ears with absent auditory nerves in MRI showed no response in the neural remote test. All patients had improved audio-speech performance with CAP scores 3.0±0.7 and 3.6±0.9, SIR scores 1.2±0.4 and 1.8±0.8, IT-MAIS scores 18.8±9.1 and 26.2±10.0, MUSS scores 2.2±2.4 and 7.2±8.3 after twelve months and twenty-four months follow up. Conclusion:Cochlear implantation in patients with CHARGE syndrome is a challenge in both its surgical and rehabilitation aspects due to multiple abnormalities. Adequate treatment planning is necessary for safe and effective surgery, including airway structures and intricate temporal bone landmarks.

Olivocochlear projections contribute to superior intensity coding in cochlear nucleus small cells

Understanding communication signals, especially in noisy environments, is crucial to social interactions. Yet, as we age, acoustic signals can be disrupted by cochlear damage and the subsequent auditory nerve fibre degeneration. The most vulnerable medium- and high-threshold-auditory nerve fibres innervate various cell types in the cochlear nucleus, among which the small cells are unique in receiving this input exclusively. Furthermore, small cells project to medial olivocochlear (MOC) neurons, which in turn send branched collaterals back into the small cell cap. Here, we use single-unit recordings to characterise small cell firing characteristics and demonstrate superior intensity coding in this cell class. We show converse effects when activating/blocking the MOC system, demonstrating that small-cell unique coding properties are facilitated by direct cholinergic input from the MOC system. Small cells also maintain tone-level coding in the presence of background noise. Finally, small cells precisely code low-frequency modulation more accurately than other ventral cochlear nucleus cell types, demonstrating accurate envelope coding that may be important for vocalisation processing. These results highlight the small cell olivocochlear circuit as a key player in signal processing in noisy environments, which may be selectively degraded in ageing or after noise insult. KEY POINTS: Cochlear nucleus small cells receive input from low/medium spontaneous rate auditory nerve fibres and medial olivocochlear neurons. Electrical stimulation of medial olivocochlear neurons in the ventral nucleus of the trapezoid body and blocking cholinergic input to small cells using atropine demonstrates an excitatory cholinergic input to small cells, which increases responses to suprathreshold sound. Unique inputs to small cells produce superior sound intensity coding. This coding of intensity is preserved in the presence of background noise, an effect exclusive to this cell type in the cochlear nucleus. These results suggest that small cells serve an essential function in the ascending auditory system, which may be relevant to disorders such as hidden hearing loss.

Animal models of hidden hearing loss: Does auditory-nerve-fiber loss cause real-world listening difficulties?

Afferent innervation of the cochlea by the auditory nerve declines during aging and potentially after sound overexposure, producing the common pathology known as cochlear synaptopathy. Auditory-nerve-fiber loss is difficult to detect with the clinical audiogram and has been proposed to cause 'hidden hearing loss' including impaired speech-in-noise perception. While evidence that auditory-nerve-fiber loss causes hidden hearing loss in humans is controversial, behavioral animal models hold promise to rigorously test this hypothesis because neural lesions can be induced and histologically validated. Here, we review recent animal behavioral studies on the impact of auditory-nerve-fiber loss on perception in a range of species. We first consider studies of tinnitus and hyperacusis inferred from acoustic startle reflexes, followed by a review of operant-conditioning studies of the audiogram, temporal integration for tones of varying duration, temporal resolution of gaps in noise, and tone-in-noise detection. Studies quantifying the audiogram show that tone-in-quiet sensitivity is unaffected by auditory-nerve-fiber loss unless neural lesions exceed 80%, at which point large deficits are possible. Changes in other aspects of perception, which were typically investigated for moderate-to-severe auditory-nerve-fiber loss of 50-70%, appear heterogeneous across studies and might be small compared to impairment caused by hair-cell pathologies. Future studies should pursue recent findings that behavioral sensitivity to brief tones and silent gaps in noise may be particularly vulnerable to auditory-nerve-fiber loss. Furthermore, aspects of auditory perception linked to central inhibition and fine neural response timing, such as modulation masking release and spatial hearing, may be productive directions for further animal behavioral research.